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I take a different position (and will probably get flamed)...
Start with 4 discs...
Buy DX or Pro <- Why? you are going to destroy all of your discs in the beginning until you get control. Why spend $20 on a disc that wont really perform much better than its $10 counterpart; better plastic does performe better, but when most of your shots are goign to hit trees anyway and YOU wont really know the differences... does it really matter? in the end a disc will be beat up super bad in 1 month anyway so the end is the same... Broken In beyond playability within 1 month.
(Also note, when you want new discs, for a while, go buy used "retreads" from the guys selling them on the course, and at the second hand outlets. A big portion of DG is learning what a disc does and what it can do for you.... When i want to experiment, i try to find used. It helps <wheoever> is selling it, and with a $5 expense, i get a broken in disc and will know what "its all about" very quickly. If i like it, i buy it new... if i dont, i keep it, give it away, or donate it to someone else.
Try to keep your discs in the 168-170 Range. As a beginner this will probably be "about right"... as your arm grows or you refine your skills you will be able to figure out what direction you need to go with the weights.
Buy a Katana in the 168 weight Range.
Yes, its bad form... yes it's against most training methods... yes it's cheating..... but why not pick up a disc that you'll be able to throw super far from day 1 (with bad technique and form). You wont be able to throw it any better than any other disc, BUT it will still go 20-40' farther from day 1. It will handle sidearm easily.
Buy a TeeBird in the 168 weight range.
This is the disc you should learn master. This is the bread and butter all "longer ranges" disc.
Buy a 173 DX Roc (5 grams or so heavier than your drivers)
The extra weight will help keep it stabilized and you will find that it can be used as a driver. It can be used for almost any shot type.
Buy a 175 Aviar / Putter
This is the standard. You can get fancy with other discs, but this is a disc you will probably have until the day you quit DG.
This selection (kinda) leaves you short with the forehand and hyzer (meat hook) type shots, but when you figure out how to throw and realize why you need that shot, AND the differences in discs, you will be set.
*My methodology is, in the beginning, get you started with easy distance, cheap equipment and something that will give you enough of a range to experiment, learn and determine if you want to get more serious. At the point you want to get more serious, you'll start to "figure it out" enough to know what you need. Until then, the main key is to get you out there, have fun, and give you the tools to ascend through the "baby steps".